Nanoimprint lithography (NIL) has emerged as a leading candidate for high throughput, high resolution nanoscale patterning due to the ability to achieve feature resolutions beyond the limits encountered by other techniques, such as optical diffraction or beam scattering . Current issues for NIL include control and removal of the residual layer, which may be critical for some applications and require subsequent process steps, and throughput, which still remains at approximately a few minutes per wafer—insufficient for the production of flat panel displays, photonics, or organic optoelectronics. Growing interest in integrated nanomanufacturing on flexible substrates for these and related applications underscores the need to develop rapid throughput NIL techniques conducive with continuous roll-to-roll processes.

Schematic of Roll-to-Roll Nanoimprint Lithography process. For high throughput processes incorporating NIL, key challenges include pressure uniformity and successful, efficient demolding in large area printing. These issues critically depend on the properties of the substrate, mold, and thermal forming resist materials. Furthermore, the residual layer thickness of the imprinted pattern must be well-controlled and repeatable to preserve pattern fidelity and maintain feature size over large areas at high process rates. Recently, Ahn, et. al., from the University of Michigan report on their investigation of large-area (4 inches wide) roll-to-roll (R2RNIL) and roll-to-plate (R2PNIL) nanoimprint lithography. Using a newly developed apparatus, the authors demonstrate significantly enhanced production of 300 nm patterned gratings on both rigid glass and flexible plastic substrates. A primary focus of this investigation was on the optimal selection of mold material, thermoepoxy resist, and the resulting residual layer thickness (RLT) as a function of key imprint process parameters, including roller pressure and speed.

The authors use a flexible fluoropolymer mold material, ethylene tetrafluoroethylene (ETFE), for its exceptional antistick properties—thereby making it relatively easy to demold after imprinting without the need for surface treatment. For fast roll-to-roll process development, the authors use a UV-curable, low-viscosity epoxysilicone as the imprint resist material. The epoxysilicone has the advantage of curing via a cationic mechanism, free from the oxygen inhibition effects and vacuum environment of UV-curing for acrylate-based resists. Additionally, the epoxysilicone exhibits very low shrinkage after curing, enabling high fidelity pattern replication. The authors demonstrate printing of 300nm linewidth gratings with 600nm heights reproducibly patterned on both types of substrates. The rapid curing of the epoxysilicone allowed for a web speed of 1 m/min, which is a significant leap over previously reported performance. Note that the epoxysilicone resist still exhibits insufficient adhesion to the glass substrate and delaminates during the demolding step, therefore requiring that adhesion promoter be applied to the substrate.

Finally, the authors describe an analytical model used to predict RLT dependence on process parameters including web speed and applied force. Incorporating an accurate description of the dynamic force applied during contact between the mold and substrate, the authors improve the accuracy of the model, exhibiting excellent agreement with experiment.

Thus, progress towards scaled, large-area NIL for high-throughput roll-to-roll and roll-to-plate processes has been demonstrated, representing a significant step towards integration of emerging nanomanufacturing techniques with high throughput production infrastructure.